6.3.8.2 The Front End config-lang.in File

Each language subdirectory contains a config-lang.in file. In
addition the main directory contains c-config-lang.in, which
contains limited information for the C language. This file is a shell
script that may define some variables describing the language:

language

This definition must be present, and gives the name of the language
for some purposes such as arguments to --enable-languages.

lang_requires

If defined, this variable lists (space-separated) language front ends
other than C that this front end requires to be enabled (with the
names given being their language settings). For example, the
Java front end depends on the C++ front end, so sets
lang_requires=c++.

subdir_requires

If defined, this variable lists (space-separated) front end directories
other than C that this front end requires to be present. For example,
the Objective-C++ front end uses source files from the C++ and
Objective-C front ends, so sets subdir_requires="cp objc".

target_libs

If defined, this variable lists (space-separated) targets in the top
level Makefile to build the runtime libraries for this
language, such as target-libobjc.

lang_dirs

If defined, this variable lists (space-separated) top level
directories (parallel to gcc), apart from the runtime libraries,
that should not be configured if this front end is not built.

build_by_default

If defined to no, this language front end is not built unless
enabled in a --enable-languages argument. Otherwise, front
ends are built by default, subject to any special logic in
configure.ac (as is present to disable the Ada front end if the
Ada compiler is not already installed).

boot_language

If defined to yes, this front end is built in stage 1 of the
bootstrap. This is only relevant to front ends written in their own
languages.

compilers

If defined, a space-separated list of compiler executables that will
be run by the driver. The names here will each end
with \$(exeext).

outputs

If defined, a space-separated list of files that should be generated
by configure substituting values in them. This mechanism can
be used to create a file language/Makefile from
language/Makefile.in, but this is deprecated, building
everything from the single gcc/Makefile is preferred.

gtfiles

If defined, a space-separated list of files that should be scanned by
gengtype.c to generate the garbage collection tables and routines for
this language. This excludes the files that are common to all front
ends. See Type Information.